For example, if a communication terminal and another end terminal use TCP/IP to make communication, an IP address is used as an address for identifying the communication terminal which is a transmission source or another end terminal which is a transmission destination. In this case, in particular, in order to make it easy to identify the communication terminal and another end terminal, there is known a method in which unique host names are assigned to the communication terminal and another end terminal and in which the host names are used to make communication (e.g. refer to patent documents 1 and 2). Specifically, for example, when the communication terminal uses the host name of another end terminal to communicate with another end terminal, a Domain Name System (DNS) server converts the host name of another end terminal to the IP address of another end terminal, and notifies the converted IP address of another end terminal to the communication terminal. Then, the communication terminal specifies the converted IP address of another end terminal as a destination address, thereby communicating with another end terminal. This makes it possible for a user of the communication terminal to communicate with another end terminal by using only the host name which is relatively easy for the user to identify.
By the way, as the IP address, there are a global address and a local address. The global address is a globally unique address. On the other hand, the local address is an address determined uniquely in a particular communication network. Moreover, a general network has a hierarchy network configuration in which a plurality of local networks using the local addresses exist in the global network using the global address. In this case, the communication using the host name described above may have the following technical problem.
Specifically, the following example is considered; namely, a “terminal A” accesses a “server A” by using the host name in such a network configuration that a local network including the “server A” with a local address of “10.0.0.123” and a host name of “AA” and the “terminal A” with a local address of “10.0.0.30” is connected to the global network via a Network Address Translation (NAT) router with a LAN side address (i.e. local address) of “10.0.0.1” and a WAN side address (i.e. global address) of “123.45.67.89” and that a DNS server with a global address of “141.140.139.138” is disposed in the global network. In this case, the “terminal A” presents the host name “AA” to access the DNS server, thereby obtaining from the DNS server an IP address for accessing the “server A”. In this case, since the DNS server merely performs the conversion from the host name to the global address, the DNS server notifies the “terminal A” of “123.45.67.89” which is the WAN side address of the NAT router included in the local network to which the “server A” belongs, as the IP address for accessing the “server A”. Then, the “terminal A” sets “123.45.67.89” notified from the DNS server as the destination address and starts the transmission of the data. However, since there is no equipment with an IP address of “123.45.67.89” in the local network, the data transmitted from the “terminal A” is transmitted to the NAT router. In this case, since the destination of the data transmitted from the “terminal A” is the WAN side address of the NAT router, the NAT router recognizes that the data transmitted from the “terminal A” is transmitted to the NAT router. As a result, the data transmitted to the “server A” from the “terminal A” is not transmitted to the “server A”.
In order to solve such a technical problem, for example, the patent document 1 discloses the configuration that a unique DNS server is disposed in each local network. Moreover, the patent document 2 discloses the configuration that the global network is provided with a relay center for further converting the global address, which is converted from the host name, to the local address.
Moreover, according to a “hairpin support router” defined in Request for Comment (RFC)-4787, the data transmitted to the WAN side address of the hairpin support router can be transmitted again to the local address.